The earliest forms of apparatus designed to heat receptacles consisted of locating a combustible source under a heat resistant receptacle. A simple example is a receptacle supporting surface located over an open fire pit. As it became more desirable to have such apparatus located in enclosed areas such as dwellings, advances in the art generated self-contained apparatus using similar heating means such as wood fired stoves which advantageously reduced the amount of combustion byproduct associated with the heat source by porting combustion byproducts to the outside of the dwelling. To this end, the combustion chamber was made distinct from the receptacle supporting surface. Only the heat, and not the combustion gasses and particulates, were transferred to the supporting surface receptacle. Thus, heating technology changed from thermal convection to thermal conduction and radiation--the hot combustion gasses heated the supporting surface which in turn heated, by thermal conduction and radiation, the receptacle.
With the advent of readily available clean burning fuels such as methane, ethane, propane, butane, and vaporized light fuels such as kerosene and alcohol, receptacle heating technology again returned to thermal convection technology. The advantages of this form of heating included the availability of "instant heat", greater thermal transfer rates per unit of energy, and low residual heat after completion of the heating process. Because the fuels produced little undesirable combustion byproducts, there was little concern over the venting of the combustion byproducts. This method, however, was not without drawbacks which included the ever present threat of accidental explosions both locally at the apparatus and in storage units or transport lines, and potential toxicity of unburned fuels.
As the utilities infrastructure of society expanded, a new and clean source of energy was made available to the public. Electricity provided a relatively safe and convenient means of providing energy needed to heat receptacles containing liquids or food. The use of electric cooktops and ovens flourished in response. As with wood stoves, however, electric cooktops relied upon thermal conduction and radiation to heat the contained liquids and food. As is well known, the receptacle support surface comprised one or more electrically resistive elements that were subjected to electric current. Resistance to the current produced radiant heat. By placing an appropriate heat conducting receptacle on the element, which doubled as a support surface, the heat generated by the element(s) would be transferred to the receptacle and thus to the contents therein.
Nevertheless, radiant heating was subject to the same limitations as the traditional wood stove, namely slow initial heat transfers, high residual heat after cessation of the electric current, and unlike wood stoves, uneven heating characteristics. In addition, radiant heating was less efficient than thermal convection heating.
From the foregoing it becomes apparent that a cooktop having the heating characteristics of thermal convection and the convenience and safety associated with electric units is most desirable.